FIG. 1 shows a portion of the active region of a trench-type power MOSFET according to the prior art. As is well known, such prior art devices include a number of spaced trenches 10 each extending through channel region 12 of the device.
Referring to FIG. 2, under reverse bias the electric field near the bottom corners of each trench 10 is high due to the crowding of the field lines. The high electric field concentration near the bottom corners of trenches 10 may result in breakdown which is an undesirable condition in that it may lead to hot carrier injection into the gate oxide, causing breakdown voltage walkout and an unreliable gate oxide.
The performance of conventional trench-type MOSFETs is sensitive to the depth of trenches. Generally, the deeper the trenches the lower the breakdown voltage and the lower the ON resistance (Rdson). Thus, in the design of trench-type MOSFETs the depth of trenches must be taken into account to obtain a device with a desired combination of Rdson and breakdown voltage rating.
An important characteristic of a trench-type MOSFET is its gate charge (Qg). The gate charge of a trench-type MOSFET is proportional to the area covered by the gate oxide 14 of the device and is naturally dependent on the depth of the trenches 10.
One component of Qg is the gate to drain charge (Qgd) which is determined by the portion of gate oxide 14 that overlaps the drain region 16 of the device. Shallow trenches, i.e., trenches that do not extend sufficiently below channel region 12, do not provide for sufficient overlap between the gate oxide 14 in the trenches 10 and the drain region 16 which is a factor contributing to the increase in Rdson. Unacceptably low overlap between the gate oxide 14 and the drain region 16 of the device may be due to the variation in the depth of trenches.
To avoid such a result, in prior art devices, the overlap of gate oxide 14 and the drain region 16 is designed to be relatively large in order to account for the trench depth variation.
For example, Qgd has been reduced in prior art devices by using a thick oxide at the bottom of the trench. To obtain the thick oxide the trench sidewalls are protected from oxidation by a material, which is then removed to grow the gate oxide on the sidewalls of the trenches. This method does not eliminate the gate to drain overlap due to trench depth variation.
It is desirable, therefore, to have a way of controlling the overlap of the gate oxide and the drain region independent of the trench depth to obtain a lower Qgd.
It is also desirable to reduce the depth of trenches without significantly increasing Rdson thus obtaining a lower Qg for the device.